Fix doc grammar and line width
Fix doc grammar and line width
_PyUnicode_Writer() now also reuses Unicode singletons:
empty string and latin1 single character
Issue #17361: Use cc from sysconfig for testing flags.
Close #15465: Document C API version macros

Mostly moving the existing macro docs over from the standard
library docs to the C API docs where they belong.

Patch by Kushal Das.